(a) Field of the Invention
The present invention relates to a method and an apparatus for disposing dust produced during the process of forming an amorphous silicon (a-Si) film, and more particularly it pertains to a method and an apparatus for disposing a large amount of dust which is produced during the formation of an a-Si film by relying on photo-CVD (Chemical Vapor Deposition) technique or plasma CVD technique.
(b) Description of the Prior Art
Since the discovery of utility of a-Si which is obtained by decomposition of silane gas by a glow discharge, there have been extensively conducted research and development in the matter of applications of and putting into practice of a-Si, and as a result a marked progress is noted at present in the improvement of products using a-Si as a constituent substance. When viewed from the standpoint of production of a-Si, however, there are not a few important problems left to be solved.
One of them concerns improvement of the deposition rate of a-Si during its production. This problem, however, has been solved already as stated in, for example, Japanese Patent Preliminary Publication Nos. Sho 59-193265 and 59-193266 of the same inventors as for the present application.
The second problem concerns the handling or disposal of the large amount of dust which is produced during the formation of a-Si films by glow discharge of silane gas. Since, in the past, a-Si itself was still in its way for development, it has been the general idea that the formation of a-Si films has been conducted only on the experimental basis in laboratories, and thus the manufacture of a-Si on a commercial mass production basis at a manufacturing plant has been very limited, if any. In the past, the disposal of dust which was produced in each experiment relied on an unsophisticated manner such that the dust was subjected to suction by using a small-sized vacuum cleaner or like means and the collected dust was discarded directly. When it is intended to make a mass-production of a-Si photoreceptors, however, the disposal of the dust becomes a very important task.
The reason therefor is due to the special physical property of silicon dust. Unless an efficient and effective method for the disposal of this dust is established, the production of a-Si will not become possible to be done on an industrial basis. As a preliminary measure for dealing with the large amount of silicon dust which entails the manufacture of a-Si, the present inventors have already solved the problem concerning a-Si film formation itself. The most important matter, however, can be boiled down to only one thing which is how the dust thus produced should be disposed. The very specific property of this dust is as follows.
(A) The dust consists of a light-weight, very fine particles of powder form which is unsteady and is ready to fly and scatter around immediately when contacted by a slight current of air, and will contaminate the environment, and thus it gives rise to a problem in the sanitary management for the workers involved in the manufacture.
(B) a-Si dust is ignited quite easily by a small source of fire such as flame of a match, lighted cigarette, sparkling, or spark from a driven rivet. The produced dust in itself has no spontaneous ignitability. Once ignited from a near by flame, however, the dust will combust furiously like fireworks, and the burning dust will easily scatter and fly around, so that this flame of buring dust will easily induce a fire on surrounding things. In case a large amount of dust is present, it will instantaneously catch fire and the flame will spread extensively throughout the area like an explosive or a firework, and accordingly the dust is very dangerous.
(C) Once ignited, the burning dust cannot be extinguished according to the present technique unless the surrounding air is completely shut off.
(D) a-Si dust has no affinity to water and the particles of dust do not disperse in water, but they remain always in the state of floating on the surface of water, and thus it is not possible to deal with the dust by using water.
(E) Accordingly, the dust, once ignited, will not become extinguished by merely sprinking water thereonto, but it will continue to burn in the state of floating on the surface of water. Also, in the presence of water, the dust will rather vigorously burn while developing a scorching heat. Thus, the burning dust would bring about the same phenomenon as if by sprinking oil onto fire if water is sprinkled onto the site of combustion of a large amount of dust.
(F) Although the dust has no affinity to water, it has affinity to organic solvents in general, and it disperses in these solvents of liquid form. However, the dust reacts with lower alcohol and produces hydrocarbon gas, silane gas and hydrogen gas, and such a gas could ignite spontaneously. Thus, the dust is dangerous, and one must avoid mixing the dust with an inflammable organic solvent.
(G) a-Si dust has a smooth flowability like a liquid. Even when dust is applied with a pressure externally, the dust will only displace around, and it is quite difficult to subject the dust to a compression pressing.
(H) Though depending on a-Si film-forming conditions employed, the dust presents a brown color in general. When the dust is combusted, the burnt dust will still remain as a powder of whitish brown color. This remnant is intensively hydrophilic, and disperses well in water, but will remain as sediments at the bottom of water when left over. The sediments are silicon oxide, which is of a same quality as soil.
As stated above, a-Si dust in its fine powder form is extremely dangerous, and is very difficult to handle. As such, the dust which is produced inevitably during mass production in an a-Si manufacturing plant will not lead to the solution of problems merely by collecting the dust which is produced. In the event that the handling or disposal of dust is not done in an appropriate way, there will arise not only problems related to environmental pollution but also a danger of destroying the surrounding things.
As will be appreciated from the foregoing statement, in the past the collection per se of dust has really been an extremely dangerous operation, and the prior method of its disposal has been just to directly combust the dust. Thus, it has not been possible to place the combustion under control, and therefore a great danger was involved. Furthermore, after combustion of dust, the remnants remain in the form of another dust or powder. Because of such dangers as mentioned above, it has been the usual technique to introduce and mix the collected dust in an amount of water containing, dissolved therein, a higher alcohol or a surface active agent to render the dust hydrophilic, and thereafter the resulting substances are dispersed in water. In such a case, it takes a considerable length of time until the burnt dust is dispersed in water. Moreover, depending on cases, the dust could develop a chemical reaction and could produce a poisonous gas. What is more, the liquid per se which contains chemical agents is difficult to dispose, and could require a very large used-liquid treating equipment which, in turn, requires a large running cost.
In case it is intended to manufacture, for example, amorphous silicon photosensitive devices, it has been the prior technique of such a low level that silane gas is supplied into the reaction chamber, while applying radio frequency (RF) to the interior of the reaction chamber to develop a glow discharge, thereby effecting deposition of a-Si substance onto the surface of a substrate which is housed therein, and that, during this reaction, the dust which is produced within the reaction chamber is collected merely by a suction means such as a rotary pump (RP), and the collected dust is discarded directly. In the past, there have been attempted various counter-measures for the danger of silane gas. With respect to the dust of silicon among others, however, the development of its counter-measure has been rather neglected so far.